The present invention relates to a novel cast iron alloy whose microstructure comprises compacted graphite and flake graphite. The invention also relates to using the novel cast iron alloy in the production of a cylinder block, a cylinder head, a bed plate, a transmission housing or an axle housing.
Cast irons are widely used for a variety of applications. The basic types of cast iron can be categorised as:
Grey cast iron, where the graphite exists as flakes or lamellar particles.
Compacted graphite iron (CGI), where the graphite particles are elongated as in grey iron but are shorter and thicker and have rounded edges and irregular bumpy surfaces.
Ductile iron, where the graphite precipitates as individual spheroids or nodules, and
Malleable iron, where the graphite particles precipitate from the solid state during heat treating operation.
The production, properties and applications of these irons is documented in, for example, the Iron Casting Handbook C. F. Walton (Editor), Iron Castings Society, and specified in the ASTM A 247 and ISO 945-1975 standards.
Until about 1960, the standards focused primarily on grey cast iron. Ideally, grey cast iron should contain long and randomly oriented graphite flakes or lamellae. However, degenerate graphite shapes may also grow under certain conditions. The grey iron terminology therefore refers to five different types of grey iron ranging from Type A to Type E. Type A graphite denotes long graphite flakes and is preferred in most applications while Types B through E are degenerate and result in lower strength. With the introduction of ductile iron in 1948, the standards were amended to include the new and different forms of graphite. The ASTM standard adopted seven different types of graphite. Type I represented ideal graphite nodules while Types II through VI showed various types of degenerate nodules. Type VII was reserved for grey iron, which then was subdivided into the established categories A through E. The ISO standard has a similar approach with only six basic forms of graphite. Form I is grey iron and Form VI represents ideal graphite nodules. Forms II through V refer to degenerate forms of nodules. Similar to the ASTM standard, ISO Form I for grey iron is sub-divided into Categories A through E to show the various types of grey iron. The definitions of A through E are common in ISO and ASTM.
As a result of the evolution of the microstructure rating standards, two entirely separate rating techniques have evolved. Grey iron is defined by reference to the different types A through E, for example, 90% Type A plus 10% Type B. Ductile iron is classified in terms of percent nodularity, that is, what percent of the graphite particles are present as perfect nodules. Commercial ductile irons must generally have more than 85% nodularity (i.e., more than 85% ASTM type I graphite or ISO Form VI graphite). Microstructure rating charts ranging from 50-100% nodularity have been widely published to assist in microscope evaluations of graphite shape.
Both the ASTM and the ISO standards include a reference to compacted graphite. Compacted graphite is represented by ISO Form III or ASTM Type IV graphite. High quality CGI should generally have more than 80% compacted graphite particles with less than 20% nodular graphite and no flake graphite. Thus, for compacted graphite iron, the industry has accepted a specification of 0-20% nodularity.
Therefore, based on the ASTM and ISO standards, a continuum has been established from perfect CGI (100% ISO Form III or 100% ASTM Type IV) to perfect ductile iron (100% ISO Form VI or 100% ASTM Type I). A nodularity scale of 0-100% is thus established, but this scale completely excludes grey cast iron. For metallurgists, grey cast iron exists on a separate xe2x80x9cA through Exe2x80x9d scale below 0% nodularity.
Until now, the vast majority of iron castings are specified in one of the above cast iron types with particular requirements for microstructural homogeneity to unify properties throughout the casting. More recently, it has been proposed that some products may benefit from the presence of different types of graphite in different areas of the casting. In this way, the mechanical and physical properties of a given type of cast iron can be exploited in the specific regions of the casting that best benefit from those properties. Specific examples include cylinder blocks that contain flake graphite or compacted graphite in the cylinder bores for heat transfer and friction behaviour and spheroidal graphite in the structural regions for rigidity and durability (EP 0769615 Al and JP 6-106331), or a flywheel that has CGI in the perimeter for machinability and spheroidal graphite in the hub for strength (WO 93/20969). Many other such examples can be cited. The concept of different graphite types in different areas of cast iron castings has not been widely accepted due to the difficulties to reliably control the production method. Indeed, it is easier to have uniform graphite throughout the casting, and easier to target the middle range of wide microstructure specifications in order to minimize foundry rejects caused by out-of-specification products. While these traditional practices facilitate foundry production, they do not always provide optimal properties and products.
In response to the ever-increasing demand for higher torque, reduced emissions and improved fuel economy, engine designers are forced to seek stronger materials for cylinder block construction. This fact is particularly true in the diesel sector where emissions and torque objectives can only be met by increasing peak cylinder combustion pressures. While today""s direct-injection passenger car diesels operate at approximately 135 bar, the next generation of DI diesels is targeting 160 bar and beyond. Peak combustion pressures in heavy duty truck applications are already exceeding 200 bar. At these operating levels, the strength, stiffness and fatigue properties of grey cast iron and the common aluminum alloys may not be sufficient to satisfy performance packaging and durability criteria. Engine designers are therefore exploring alloyed grey cast irons and CGI to extend the operating range of their designs. In many cases, the strength of alloyed grey iron may not be sufficient while that of CGI may be more than is required. Conventional (5-20% nodularity) CGI can also be prone to shrinkage defects in complex castings.
In addition to strength limitations of approximately 300 MPa, alloyed grey irons are difficult to machine and frequently crack during shake-out, cooling and handling. The high alloy content also restricts recycling of returns within the foundry.
While 5-20% nodularity compacted graphite iron has more-than-adequate strength, it""s application may be limited by machining, particularly the high speed cylinder boring operation. The thermal conductivity of CGI, being approximately 20% lower than that of grey iron can also be problematic in some designs. Another problem that may occur when casting CGI is shrinkage. A casting that has undergone shrinkage may have internal porosity or surface depressions requiring that these castings be discarded. Even worse, the internal shrinkage porosity may not be observed during quality inspection and finished products made from such castings would be prone to premature failure in service.
Accordingly, there is a need for a material which is sufficiently strong to fulfil the increased strength demand, and which is less prone to shrinkage.
When the magnesium treatment of compacted graphite iron is insufficient to stabilize a fully compacted graphite morphology, the graphite may begin to grow with a flake graphite morphology. As the solidification of each eutectic cell progresses radially outward, the magnesium concentration segregates ahead of the solidification front. The magnesium may become sufficiently high to stabilize compacted graphite iron around the perimeter of the eutectic cell. The resultant microstructure is referred to herein as flake-patch CGI (FIG. 1). It is well known that these flake patches cause a precipitous decrease in the tensile strength and stiffness of CGI. For this reason, several authors have clearly shown that flake patches must be avoided in castings designed for CGI (C. R. Reese and W. J. Evans: Development of an inmold treatment process for compacted graphite iron cylinder blocks, AFS Annual Foundry Congress, Atlanta, 1998. Also, R. J. Warrick et al: Development and application of enhanced compacted graphite iron for the bedplate of the new Chrysler 4.7 liter V-8 engine, SAE Paper No. 99P-144).
It has now turned out that the above mentioned strength and shrinkage problems can be solved by providing a cast iron alloy having the following characteristics:
A representative chemical specification for such an alloy is 3.0-3.8% carbon, 1.6-2.5% silicon, 0.2-0.65% manganese, 0.01-0.1% tin,  less than 0.025% sulfur, 0.001-0.020% magnesium, 0.1-1.2% copper, 0.04-0.2% chromium, and balance up to 100% of iron.